1. Field of the Invention
The present invention relates to acetabular cups and, more particularly, to acetabular cup assemblies for use with multiple bearings.
2. Related Art
In hip arthroplasty, various bearing materials are available for the acetabular cup portion of an implant. The selection of the bearing material is typically determined by the surgeon prior to performance of the procedure. At times, however, final selection of the bearing material is not completed until the implant site is prepared and conditions at the site are evaluated. Thus, it is sometimes advantageous to utilize an acetabular shell that accepts multiple bearing liners so that the surgeon can revise the initial assessment if required.
Acetabular shells that accept multiple bearings have been used in Europe since the early 1980s. Typically, the outer shell featured a tapered inside geometry in which a tapered hard or soft bearing could be inserted. In these cases, soft bearing thickness, lock integrity and wear performance were compromised in an effort to accommodate both bearings.
More recently, Stryker Corp. of Kalamazoo, Mich., U.S.A., has developed an acetabular cup that accepts a fully metal-encapsulated ceramic insert, which is held via a taper lock connection. The shell also accepts a polyethylene insert that is locked via one circumferential bead located mid-point along the inner taper and is rotationally stabilized by four conforming features between the shell and the liner.
An acetabular cup assembly for use with multiple bearings is desirable from a manufacturing standpoint because it is only necessary to produce one shell for use in many applications. This increases the volume of shells produced, which decreases overall production costs. Further, production of a single shell reduces distribution costs.
An acetabular cup assembly for use with multiple bearings is desirable from a revision standpoint because it gives the surgeon greater flexibility and reduces the overall time of the operation. First, the assembly gives the surgeon greater flexibility because the surgeon can easily make adjustments to the hip prosthesis. For example, if the original prosthesis had a polyethylene liner, the surgeon can easily substitute a ceramic or metal liner without changing the shell. Second, the assembly reduces the overall operation time because it is not necessary to remove the shell. Typically, the installed shell is surrounded by ingrown bone, which is very difficult and time consuming to remove. Further, removal of the installed shell may result in significant bone loss. By eliminating the step of removing the shell, the surgeon can complete the revision in less time with less effort and the result is less traumatic to the patient.
Micromotion between a polyethylene liner and an acetabular shell is undesirable as the motion creates polyethylene debris, which eventually causes bone osteolysis. Prior polyethylene bearing lock mechanisms were designed to exhibit minimal micromotion between the liner and the shell. However, these mechanisms also required an excessive interoperative insertion force for insertion of the liner. A high insertion force is undesirable as it requires greater effort on behalf of the surgeon to install the liner.
Traditionally, ceramic liner manufacturers have advised against reinsertion of ceramic liners due to the stress-sensitive nature of the material. The material may fracture or break if stressed inappropriately. However, for various reasons, it may be desirable to remove and reinstall a liner. As an example, a surgeon may want to remove the ceramic liner during installation, change the shell position, and reinstall the liner. As ceramic manufactures presently advise against this, a surgeon takes on great risk when making these types of adjustments during ceramic liner installations.
There remains a need in the art for an acetabular cup assembly for use with multiple bearings.